JPH04310836A - Method for measure distribution of refractive index - Google Patents

Abstract

PURPOSE:To enable a measurement shape of distribution of a refractive index to be measured highly accurately and easily by increasing positioning accuracy of a measurement sample. CONSTITUTION:An interposed member 6 is placed between a measurement sample 5 and a sample-installation side surface 4b of a medium member 4 and the interval between the both is so set that the interposed member has a thickness preventing an adhesion force between the measurement sample and the medium member. Therefore, by performing coating on the sample- installation side surface 4b or by adopting the medium member 4 and the interposed member 6 with mutually small difference in the refractive index, the reflectivity of the boundary surface between the medium member 4 and the interposed member 6 is reduced and the contrast of interference fringes which is generated in reference to a reflection light on the boundary surface between the measurement material 5 and the interposed member 6 including brightness/ darkness boundary information is reduced, thus enabling the brightness/darkness boundary to be detected.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a refractive index distribution measuring method for measuring the refractive index distribution shape of an optical element having a refractive index distribution using total reflection of light.

0002

2. Description of the Related Art In recent years, refractive index distribution type optical elements having a refractive index distribution have been widely used as pickups in video disc devices and array lenses in copying devices. Furthermore, in the field of optoelectronics, relatively small graded refractive index optical elements such as optical waveguides and flat plate microlenses with graded refractive index are used; in the field of imaging, relatively large diameter optical elements are used as lens systems for silver halide cameras, video cameras, microscopes, etc. Various refractive index gradient optical elements are being put into practical use.

The characteristics of these gradient index optical elements are largely dependent on the state of their refractive index distribution, and therefore, for practical use, a method is required that can measure the shape of the refractive index distribution in each element with high precision.

Conventionally, a method for measuring such a refractive index distribution is to observe a thin sample cut and polished in a direction perpendicular to the central axis of the refractive index distribution using an interference microscope, and to calculate the per unit thickness of the thin sample. The refractive index distribution can be measured by longitudinal interferometry, which measures the refractive index distribution by determining the optical path length difference, or by transmitting a ray in a direction perpendicular to the central axis of the refractive index distribution of a cylindrical measurement sample and tracing the ray. A lateral interferometry method is known to find the .

[0005] More recently, Japanese Patent Application Laid-Open No. 63-275936
A measurement method as described in the above publication has also been proposed. This method applies the principle of Pulfrich's refractometer, which is a well-known technique, and as shown in FIG. The measurement point 1 on the measurement stage installation surface 102A is placed in close contact with the sample installation surface 102A, and is captured by the condenser lens 103 through the hemispherical surface 102B other than the installation surface 102A.
This is done by inputting a laser beam 105 that is converged onto the laser beam 04. Of the convergent light irradiated to the measurement point 104, the light in the luminous flux region that enters at the point 104 at an incident angle range larger than the total reflection critical angle φc is totally reflected, so it has almost the same brightness as the incident light. The reflected light of the light region 106 is obtained, and a portion of the light in the luminous flux region that enters at an incident angle range smaller than the total reflection critical angle φc is transmitted and emitted from the measurement point 104, so the light region is darker than the incident light. 107 reflected lights are obtained. Therefore, when the light beam reflected from the measurement point 104 is observed, a cross section 109 of the light beam divided into a relatively bright light region 106 and a relatively dark light region 107 across the bright/dark boundary 108 can be measured. In this way, the incident light having the critical angle of total reflection φc is reflected as the bright and dark boundary 108, which is the boundary between the bright and dark areas, so if we measure the angle that the light ray at the bright and dark boundary 108 makes with the normal to the measurement surface, , its value is the total internal reflection critical angle φc,
The refractive index n of the measurement sample 101 at the measurement point 104 is determined from the known refractive index no of the measurement table 102 using the following equation (1). n=no sinφc
(1) Furthermore, the measurement sample 101 is placed on the measurement table 1.
If the measurement sample 10 is moved horizontally while keeping it in close contact with
The refractive index distribution shape of 1 is found.

[0006]

[Problems to be Solved by the Invention] However, each of these conventional measurement methods has different problems. First, although both longitudinal and transverse interferometry have high detection accuracy, in the case of the former, the larger the difference in refractive index distributed in the measurement sample, the more accurate the measurement becomes unless the measurement sample is cut into thin pieces and polished. However, depending on the object being measured, the
Some samples had to be processed into extremely thin measurement samples, which were as thin as 1.5 m, making it very difficult to prepare the measurement samples, and because the samples were cut and polished, they had to be constantly destroyed in order to be able to be measured. In the latter case, although there is an advantage that the measurement sample can be measured non-destructively, there are disadvantages such as the shape of the measurement sample is limited to a cylindrical shape and it takes time to measure the refractive index distribution.

Next, although the method described in Japanese Patent Application Laid-Open No. 63-275936 has a simple structure and is an effective method that almost eliminates the drawbacks of the above two measurement methods, In order to obtain the refractive index distribution at one point of the converged incident light beam, it is necessary to scan two-dimensionally to obtain the refractive index distribution, and the scanning accuracy greatly affects the measurement accuracy of the refractive index distribution. It turns out. In particular, the measurement sample 101 is passed through the matching refractive liquid to the measurement table 102.
Therefore, the measurement surface of the measurement sample 101 must be scanned following the planar sample installation surface 102A of the measurement stand 102 as a guide, and the measurement surface of the measurement sample 101 and the measurement stand must be scanned. High processing accuracy is required for the sample installation surface 102A of 102. Also, due to the viscosity of the matching refractive liquid (matching oil), the measurement sample 1
Since a large adhesion (adsorption) force acts between the measurement surface 01 and the sample installation surface 102A of the measurement table 102, it is actually quite difficult to configure them as a scanning system with high positioning accuracy. However, there was a drawback that it was difficult to measure the refractive index distribution of the measurement sample with high precision.

In view of the above problems, it is an object of the present invention to provide a refractive index distribution measuring method that allows the refractive index distribution shape of a refractive index distribution type optical element to be easily and highly accurately measured.

[0009]

[Means for Solving the Problems] A method for measuring refractive index distribution according to the present invention is to provide a method for measuring a refractive index distribution on one side of a measurement sample having a refractive index distribution through an intervening member having a refractive index higher than that of the measurement sample.
The sample-installed side surface of a medium member with a higher refractive index than the measurement sample is arranged almost parallel to the sample-installed side surface of the medium member, and the reflectance of the interface between the sample-installed side surface of the medium member and the intervening member is set to be low, and the thickness of the intervening member is The size shall be set to a predetermined size so that almost no adhesion force acts between the side surface of the medium member and the sample installation side, and electromagnetic waves of a predetermined wavelength are used as convergent light to pass through the above-mentioned medium member and intervening member to the measurement position of the measurement sample. The light and dark boundary of the reflected light due to total reflection at the interface between the intervening member and the measurement sample is detected, the total reflection critical angle at this measurement position is measured, and the distance between the measurement sample and the medium member is kept constant. The refractive index distribution of the measurement sample is obtained by repeating the above-mentioned process while scanning the measurement sample relative to the convergent light while maintaining the convergent light.

0010

[Operation] According to the above-mentioned configuration, when the measurement sample is scanned, the adhesion force due to the viscosity of the intervening member located between the measurement sample and the medium member hardly acts, so that the relative movement of the measurement sample is smooth. Since the measurement sample can be easily moved in parallel to the sample installation side surface of the medium member, a scanning system with high positioning accuracy can be constructed, and a highly accurate measurement of the refractive index distribution can be easily performed.

[0011] It should be noted that if the thickness of the intervening member is set to such a value that the adhesion force of the intervening member that may occur between the measurement sample and the medium member as described above will hardly act, it is advantageous in terms of positioning accuracy. , interference occurs between the light reflected at the interface between the sample installation side of the medium member and the intervening member, and the light reflected at the interface between the intervening member containing brightness boundary information and the measurement sample, resulting in a contrast near the brightness and darkness boundary. Since high interference fringes are generated, the detection accuracy of bright and dark boundaries is reduced. On the contrary,
By lowering the reflectance of convergent light at the interface between the medium member and the intervening member and lowering the intensity of the reflected light flux at this interface, the contrast of interference fringes can be lowered and bright and dark boundaries can be detected precisely. Can be done.

0012

[Embodiment] A first preferred embodiment of the present invention will be described below with reference to FIG. FIG. 1 shows a measurement optical system for carrying out the method according to the present invention, and in the figure, 1 indicates a wavelength of 632.
．． 8nm He-Ne laser, wavelength 488nm Ar
The light source includes a laser, a Kr laser with a wavelength of 568.2 nm, and a He-Cd laser with a wavelength of 441.6 nm, and a switching mirror (not shown) selects and emits an appropriate laser beam depending on the object to be measured. There is. 2 is a beam converting member into which this laser beam is incident, and after the incident laser beam is expanded to about 10 times by an objective lens with a magnification of 50 times and an objective lens with a magnification of 5 times (both not shown), the working A 20x objective lens (not shown) with a long distance (distance from the tip of the lens to the measurement point) converts the incident light beam into a converging incident light beam 3, which can be focused on the measurement point.

4 has a surface 4a on which the incident light beam 3 enters, for example, a spherical surface with a radius r=8 mm, and a side surface 4b on which the sample is placed.
is a medium member formed substantially flat, for example, He-N
It is made of a glass material having a refractive index of 1.87852 and an Abbe number of 40.8 when e-laser light is used. 5
6 is an optical element to be measured, that is, a measurement sample placed on the sample installation side surface 4b, and 6 is a matching refractive liquid, that is, an intervening member located between the sample installation side surface 4b of the medium member 4 and the measurement sample 5. For example, He -Ne has a refractive index of 1.693 and an Abbe number of 19, and its thickness is equal to the distance between the sample installation side surface 4b of the medium member 4 and the measurement sample 5. It is assumed that the magnitude is such that when the intervening member 6 is moved relative to the incident light beam 3, the adhesion force due to the viscosity of the intervening member 6 hardly acts between the medium member 4 and the measurement sample 5. According to experimental results, this interval is preferably 3 .mu.m or more, and more preferably 100 .mu.m or less in consideration of liquid dripping and the like. In this embodiment, the thickness is 5 μm. Note that the refractive indexes of the medium member 4 and the intervening member 6 are both higher than the refractive index of the measurement sample 5.

7 is the spherical center of the spherical surface 4a of the medium member 4, which is on the measurement surface of the measurement sample 5 located at the interface with the intervening member 6 and corresponds to the ideal measurement point on which the incident light beam 3 should converge. Therefore, the medium member 4 is not completely hemispherical,
Its wall thickness is reduced by the thickness of the intervening member 6. 8
is a scanning means having an X-Y stage, a tilt stage, etc. (not shown) for two-dimensionally scanning the measurement sample 5 and keeping the distance between the sample installation side surface 4b and the measurement sample 5 substantially constant; is relatively moved along the sample installation side surface 4b of the medium member 4 while maintaining a constant distance from the sample installation side surface 4b. Reference numeral 9 denotes a reflected light beam obtained by reflecting the incident light beam 3 on the measurement surface, and 10 indicates a measurement point, that is, a photodetector is arranged at a distance of approximately 50 mm from the spherical center 7 of the medium member 4 and is rotatable around the spherical center 7. This is an observation means that measures the critical angle of total reflection by receiving the reflected light beam 9 at a position where the received electromagnetic wave intensity changes most rapidly, that is, the angle between the reflected light beam 9 at the boundary between bright and dark and the normal to the measurement surface. It can be measured as the critical angle φc. 11 is a calculation means for calculating the refractive index of the measurement point 7 in the measurement plane of the measurement sample 5 from the measured total reflection critical angle φc using the above equation (1); It also has the function of capturing position information and processing it together with refractive index information to calculate and output the refractive index distribution shape of the measurement sample.

By the way, by setting the distance between the medium member 4 and the measurement sample 5 with the intervening member 6 to such an extent that no adhesion force is exerted, the sample installation side surface 4b of the medium member 4 can be
The light beam reflected at the interface between the intervening member 6 and the intervening member 6
Since interference occurs with the light beam containing bright/dark boundary information reflected at the interface between the light and the measurement sample 5, high-contrast interference fringes are generated near the bright/dark boundary, which adversely affects detection accuracy of the bright/dark boundary. In this embodiment, as a means for improving this, a means is proposed that lowers the reflectance at the interface between the sample installation side surface 4b of the medium member 4 and the intervening member 6 to lower the contrast of the interference fringes.

That is, by applying a multilayer coating to the sample installation side surface 4a of the medium member 4, the reflectance at the boundary with the intervening member 6 can be lowered. The coating is
1 mm in diameter from the center on the sample installation side 4b of the medium member 4
The thickness of Al2 03 is 36 nm, Y2 03
101 nm thick, Al2 03 85 nm thick,
Y2 03 with a thickness of 78 nm, Al2 03 with a thickness of 9
4 nm and Y2 03 are sequentially laminated to a thickness of 38 nm, and is configured as a 6-layer coat 13.

Since this embodiment is constructed in this manner, the laser beam emitted from the light source 1 and converted into convergent light 3 by the light flux converting member 2 is directed to the spherical surface of the medium member 4, as in the prior art described above. 4a and the intervening member 6, it converges at the measurement point 7 of the measurement sample 5, becomes a reflected light beam 9, and is received by the observation means 10. Here, the reflectance of the convergent light 3 incident on the interface between the sample installation side surface 4b of the medium member 4 and the intervening member 6 is lowered by the six-layer coating 13 on this surface. Therefore, the contrast of the interference fringes generated by the reflected light here and the reflected light containing bright/dark boundary information reflected at the interface between the intervening member 6 and the measurement sample 5 decreases, so that the bright/dark boundary cannot be detected precisely. Can be done. The reflected light beam 9 received by the observation means 10 is calculated and measured as a total reflection critical angle φc, and is calculated together with the measurement position information from the scanning means 8 to output refractive index information. Furthermore,
When the measurement sample 5 is scanned by the scanning means 8, the adhesion force due to the viscosity of the intervening member 6 hardly acts, so the distance between the sample installation side surface 4b and the measurement surface of the measurement sample 5 is kept constant. 5 can be moved smoothly and the refractive index distribution shape can be detected by repeating measurements while relatively scanning the measurement sample 5.

FIG. 2 shows the results of a simulation regarding the effect of the six-layer coating 13. The broken line shows the reflectance when using He-Ne laser light with a wavelength of 632.8 nm when no coating is applied to the sample installation side surface 4b, while the reflectance of the light source 1 when the coating according to this example is applied is shown. The respective reflectances when using laser beams of four different wavelengths are shown by other lines. Comparing the two, the reflectance when the coating was applied is considerably lower, giving a good understanding of the effectiveness of the coating. In addition, in the figure, the angle taken on the horizontal axis is
The reflected light beam 9 from the boundary between the sample installation side surface 4b of the medium member 4 and the intervening member 6 is the angle made with the normal line of the sample installation side surface 4b, and the case of circularly polarized light is shown.

Next, as an example, we simulated the state of interference fringes that appear near the edge, that is, near the bright and dark boundary when using Ar laser light with a wavelength of 488 nm. Figure 3 shows the case without coating, and the case with coating. are now shown in FIG. In the case without coating as shown in Fig. 3, the edge is buried in interference fringes and cannot be detected, but if coating is applied, as shown in Fig. 4, although some interference fringes remain, the contrast of the interference fringes is lower than that of the edges. It can be seen that the bright and dark boundaries can be detected sufficiently clearly. The angle shown on the horizontal axis in FIGS. 3 and 4 is the angle that the incident light beam 3 on the boundary between the sample installation side surface 4b of the medium member 4 and the intervening member 6 makes with the normal line of the sample installation side surface 4b,
The case of circularly polarized light is shown. Further, in both figures, the refractive index at the measurement point of the measurement sample 5 is set to 1.6.

As described above, according to this embodiment, the relative movement of the measurement sample 5 by the scanning means 8 is smooth, and the X-Y stage and tilting stage of the scanning means 8 are also moved by the intervening member 6.
Moreover, the intervening member 6
Since the contrast of interference fringes can be suppressed to a low level, high positioning accuracy can be achieved simply by using a simple scanning means such as a standard stage with low manufacturing cost.

Next, a second embodiment will be described for improving interference fringes with strong contrast that occur near the bright-dark boundary. In this embodiment, by reducing the difference in refractive index between the medium member 4 and the intervening member 6, the reflectance at the interface between the two is reduced.

As an example, He-
Medium member 4 and intervening member 6 according to the wavelength of He-Ne laser light and the wavelength of He-Cd laser light when using a glass material with a refractive index of 1.69426 at the wavelength of Ne laser light and an Abbe number of 30.1 Figure 5 shows the reflectance at the interface between
are shown by a solid line and a dashed-dotted line, respectively. Here, the reflectance shown by the solid line is the He-
The refractive index at the wavelength of the Ne laser beam is 1.6931, and the difference in refractive index with the medium member 4 is about 0.001. In addition, the reflectance indicated by the dashed line is He-
The refractive index of the medium member 4 at the wavelength of the Cd laser beam is 1.7.
2742, the refractive index of the matching refractive liquid is 1.72733
(This is a matching refractive liquid different from the one described above), and the difference in refractive index between the two is about 0.0001. Furthermore, for comparison, FIG.
The broken line shows the reflectance at the interface of the medium member 4 which is not coated, which is shown by the broken line, and in this case, the medium member 4 has a refractive index of 1.87852 at the wavelength of the He-Ne laser beam. The matching refractive liquid, which is the intervening member 6, also has a refractive index of 1.693 at the wavelength of the He-Ne laser beam.

It can be seen that this embodiment is more effective in reducing the reflectance at the interface between the medium member 4 and the intervening member 6 than the conventional method. Furthermore, Figure 5
The angle shown on the horizontal axis is the angle that the emitted light, that is, the reflected light, from the interface between the sample installation side surface 4b of the medium member 4 and the intervening member 6 makes with the normal line of the sample installation side surface 4b, and in the case of circularly polarized light. It is shown.

Next, in the same manner as in the first embodiment, the results of a simulation of interference fringes appearing near the edge, that is, near the bright/dark boundary, are shown in FIGS. 6 to 8 based on the above-mentioned reflectances. First, FIG. 6 shows the case indicated by the broken line in FIG. 5, where the difference in refractive index between the medium member 4 and the intervening member 6 depending on the wavelength of the He-Ne laser beam is about 0.2. In this case, the edge becomes an interference fringe. It is buried and the bright/dark boundary cannot be detected. Also, Figure 7
This is the case shown by the solid line in FIG. 5 where the refractive index difference is approximately 0.001, and edges can be sufficiently detected. In addition, Fig. 8 shows He-C
This shows the case of the dashed dotted line in FIG. 5 in which the difference in refractive index depending on the wavelength of the d laser beam is about 0.0001, and in this case as well, edges can be detected sufficiently.

The angle shown on the horizontal axis in FIGS. 6 to 8 is the angle that the incident light on the interface between the sample installation side surface 4b of the medium member 4 and the intervening member 6 makes with the normal line of the sample installation side surface 4b. , and the case of circularly polarized light is shown. Furthermore, these three data are based on the assumption that the refractive index at the measurement point of the measurement sample 5 is 1.6.

Furthermore, although each medium member 4 described above has a refractive index larger than the refractive index of the intervening member 6, it may have a refractive index smaller than the refractive index of the intervening member 6. . Regarding the refractive index difference between the medium member 4 and the intervening member 6, as a result of simulations using various numerical values, it is not limited to the numerical examples mentioned above, but if the refractive index difference is about 0.05 or less, interference fringes will occur. It seems possible to obtain the effect that edges can be detected by lowering the contrast of the image.

In each of the embodiments described above, the medium member 4 having a refractive index having a small difference from the refractive index of the intervening member 6 is used.
This is because matching refractive liquids with high refractive indexes have unstable properties such as volatilization and coagulation, and are toxic, making them difficult to handle.
This is because a matching refractive liquid having stable characteristics and a relatively low refractive index within a harmless range is used. Therefore, in principle, the medium member 4 with a high refractive index may be used and the intervening member 6 with a refractive index similar to this may be used. For example, such a combination may be used in a controlled closed room. It is also possible to adopt

Further, in each of the above embodiments, a coherent light source using a laser is used as the light source 1, but an incoherent light source such as a Xe lamp may be used after being separated by a monochromator, or an emission line light source may be used. Good too. Further, instead of the photodetector as the observation means 10, a CCD array, a CCD camera, etc. may be used, or visual observation may be performed.

[0029]

Effects of the Invention As described above, the refractive index distribution measuring method according to the present invention involves interposing an intervening member having a thickness such that almost no adhesion force acts between the measurement sample and the medium member, and By reducing the reflectance of light at the interface between the surface and the intervening member, positioning during scanning of the measurement sample becomes extremely precise, and the contrast of interference fringes near the bright-dark boundary that may occur during measurement is reduced. By lowering the value, it is possible to prevent the detection accuracy of the bright-dark boundary from deteriorating, and it is possible to easily measure the refractive index distribution shape of the measurement sample with high precision.

[Brief explanation of drawings]

FIG. 1 is a principle diagram showing an embodiment of the refractive index distribution measuring method according to the present invention.

FIG. 2 is a diagram showing the effect of reducing reflectance at the interface between the medium member and the intervening member in the first embodiment.

FIG. 3 is a diagram showing how interference fringes appear near the bright-dark boundary when no coating is applied.

FIG. 4 is a diagram showing how interference fringes appear near the bright/dark boundary when coating is applied.

FIG. 5 is a diagram showing the effect of reducing reflectance at the interface between the medium member and the intervening member in the second embodiment.

FIG. 6 is a diagram showing interference fringes that appear near the bright-dark boundary when the refractive index difference is 0.2.

FIG. 7 is a diagram showing how interference fringes appear near the bright-dark boundary when the refractive index difference is 0.001.

FIG. 8 is a diagram showing how interference fringes appear near the bright-dark boundary when the refractive index difference is 0.0001.

FIG. 9 is a principle diagram of a conventional refractive index distribution measurement method.

[Explanation of symbols]

1 Light source 4 Medium member 5 Measurement sample 6 Intervening member 7 Measurement point 10 Observation means

Claims (1)

[Claims] Claim 1: A sample-installed side surface of a medium member having a refractive index higher than that of the measurement sample is placed approximately parallel to one surface of a measurement sample having a refractive index distribution through an intervening member having a higher refractive index than the measurement sample. and set the reflectance of the interface between the sample installation side of the medium member and the intervening member to be low, and set the thickness of the intervening member to be between the sample installation side of the medium member and the measurement sample. The size is set to such an extent that almost no adhesion force is exerted, and electromagnetic waves of a predetermined wavelength are made to enter the measurement position of the measurement sample through the medium member and the intervening member as convergent light, and the entire interface between the intervening member and the measurement sample is The brightness boundary of the reflected light due to reflection is detected to measure the critical angle of total reflection at the measurement position, and the distance between the measurement sample and the side surface of the medium member on which the sample is installed is kept constant with respect to the convergent light. A refractive index distribution measuring method characterized in that the refractive index distribution of a measurement sample is determined by relatively scanning the measurement sample and repeating the above process.